There is a known tapping screw which is screwed into a guide hole formed in a to-be fastened member to form an internal thread on a surface of the guide hole by plastic deformation. As such a tapping screw, there is a known technique in which a projection for forming an internal thread on a region where a taper thread is provided, and an internal thread with which a parallel screw is threadedly engaged is formed in a surface of a guide hole by this projection (see patent documents 1 to 3). There is also known a technique in which a shank is provided at its tip end with a substantially cylindrical working screw thread having a diameter larger than that of a standard screw thread by an amount corresponding to a spring back (see patent document 4).
In such tapping screws, a external thread of the parallel screw (standard screw) is threadedly engaged with the internal thread formed by the projection during a screwing motion. Therefore, if the number of screw threads of the external thread which is screwed into the internal thread is increased, the frictional resistance is increased and thus, the screwing torque is increased in proportion to the screwing depth. Thus, when it is necessary to screw the external thread into a deep guide hole, the screwing torque becomes excessively high and the operability is deteriorated. With this, a sufficient axial force can not be obtained, and there is an adverse possibility that the fastened state becomes insufficient.
In the case of the conventional tapping screws, even if a bolt having the same nominal diameter as that of the internal thread formed by the tapping screw is screwed into the hole, since the inner diameter of the internal thread is small, the external thread and the internal thread interfere with each other. Thus, after a tapping screw which is once fastened is disengaged, if a general bolt is fastened to the internal thread formed by the tapping screw, a stable fastening axial force can not be obtained unlike general bolt and nut.
Patent document 1: Japanese Patent Publication No. S62-55005
Patent document 2: Japanese Patent Publication No. S39-14383
Patent document 3: Japanese Patent Application Laid-open No. H8-338412
Patent document 4: Japanese Patent Application Laid-open No. 2000-35016